Conventionally, a device commonly used as a pressure-sensitive sensor is constructed such that four electrode wires are circumferentially disposed on the surface of the inner circumference of a restorable hollow insulator at predetermined intervals where respective electrode wires are longitudinally disposed in a spiral manner (for example, see Japanese Patent No. 3354506 (patent literature 1)). The pressure-sensitive sensor described in patent literature 1 is constructed such that a pair of electrode wires (two wires) of the above four electrode wires is connected to a resistive element via a support member on the base end side of the pressure-sensitive sensor. Due to this configuration, electric current which was flowing through the resistive element in a state where a pressing force was not applied to the pressure-sensitive sensor flows without passing through the resistive element once a pressing force is applied to the pressure-sensitive sensor and at least two electrode wires out of four electrode wires contact one another (short circuit). The pressure-sensitive sensor described in patent literature 1 is capable of sensing the presence or absence of a pressing force by sensing the presence or absence of a resistance value of the resistive element.
However, recently, as the mounting portion of the pressure-sensitive sensor becomes small, it is required that the pressure-sensitive sensor becomes further smaller. Moreover, as with other sensors, cost reduction has been required with regard to the pressure-sensitive sensor.
To meet such requirements, it has been proposed that the outer diameter of the pressure-sensitive sensor itself be made small by reducing the number of electrode wires from four to two to make the sensor compact and also a further reduction of cost be attempted by reducing the amount of electrode wires to be used (for example, see published unexamined Japanese Patent Laid-open No. 2005-302736 (patent literature 2)).
In the pressure-sensitive sensor having two electrode wires described in patent literature 2, it is preferable that a resistive element be connected to the tip of the electrode wires so as to effectively exert the sensor function in its longitudinal direction. However, since the portion to which the resistive element has been mounted is not sensitive to a pressing force (hereafter, the portion which is not sensitive to the pressing force is referred to as “non-pressure-sensitive portion”), it is necessary to reduce the size of the non-pressure-sensitive portion by as much as possible so as to effectively utilize the function throughout the pressure-sensitive sensor including its end portion. This means that the resistive element needs to be mounted to an area that is as small as possible. Connecting the electrode wires at the tip portion to the lead wires of the resistive element by the use of the support member described in patent literature 1 can be considered; however, since the above-mentioned support member itself is wide and long, the size of the non-pressure-sensitive portion of the pressure-sensitive sensor increases. For this reason, it is desirable that the electrode wires and the lead wires of the resistive element be directly connected without providing a support member.
A possible connection method is, for example, one that uses ultrasonic vibration, which is described in published unexamined Japanese Patent Laid-open No. 2004-220933 (patent literature 3). In the connection method described in patent literature 3, a plurality of electric wires are disposed between an ultrasonic horn and an anvil, and a compression force and an ultrasonic vibration are applied to the plurality of electric wires thereby welding together the plurality of electric wires.
Furthermore, another possible connection method is, for example, one that uses resistance welding, which is described in published unexamined Japanese Patent Laid-open No. 2003-162933 (patent literature 4). In the connection method described in patent literature 4, the electrode wires of the pressure-sensitive sensor and the in-car side cable are connected by the resistance welding via a wiring pattern created on the insulating base.
However, when the connection method described in patent literature 3 is used, there is a problem in that it is not easy to produce a pressure-sensitive sensor having two electrode wires. That is, the connection method described in patent literature 3 is a method in which a groove is formed in a predetermined position of an anvil, and a plurality of electric wires is inserted into the inside of the groove to make connections. In this method, when two electrode wires are disposed very close to each other so as to prevent the increase in the size of the pressure-sensitive sensor, the side wall of the anvil becomes an obstacle, which makes it difficult to connect the electrode wires and the resistive element. Due to this, there was a problem in that it is not easy to produce a pressure-sensitive sensor having two electrode wires.
Furthermore, when the connection method described in patent literature 4 is used, there is a problem in that the size of the non-pressure-sensitive portion of the pressure-sensitive sensor having two electrode wires increases. This means that in the connection method described in patent literature 4 it is necessary to appose a pair of positive and negative electrodes for resistance welding on the wiring pattern at the time of resistance welding. This requires a space where positive and negative electrodes can be disposed on the wiring pattern; therefore, it is inevitably necessary to make the wiring pattern large. Consequently, when the connection method described in patent literature 4 is applied to the pressure-sensitive sensor having two electrode wires, there is a problem that the size of the non-pressure-sensitive portion increases.
In light of the above circumstances, an objective of the present invention is to provide a pressure-sensitive sensor production method and a pressure-sensitive sensor that has two electrode wires to contribute to reducing costs and the size and is capable of preventing the increase in the size of the non-pressure-sensitive portion and achieving easy production.